1. Field of the Invention
The present invention relates to a metal plating method for depositing a metal such as chromium on the surface of an object body to be plated which is immersed in a plating solution by electroplating by pulsed electrolysis.
2. Description of the Prior Art
Chromium plating has conventionally been carried out to obtain a highly corrosion resistible hard coating. In this case, since cracking is likely to occur on the surface of the object body to be plated if chromium is plated directly on the surface, nickel plating is carried out on the surface of the object body to make the surface of the foregoing object body smooth and then chromium plating is carried out. That is, in general, the highly corrosion resistant hard chromium coating has a double layer structure of nickel and chromium.
The foregoing chromium deposition is carried out by depositing a chromium layer on the surface of an object body to be plated by applying direct current while immersing the object body in a plating solution in a plating tank. Electrolysis is generally carried out by continuously applying direct current of 10 to 60 A/dm2. The temperature of the plating solution bath is about 40 to 60xc2x0 C.
With the foregoing electroplating method, the coating thickness of the resulting chromium layer cannot be about 10 xcexcm or thinner and when attempting to make the coating thicker, cracking may occur resulting poor corrosion resistance.
Further, there also occurs a problem that the luster of the plated film is inferior.
The foregoing cracking occurrence is attributed to stress generated by hydrogen evolved simultaneously with chromium electrode position. In other words, at the time of reductive precipitation, about 8 to 10 hydrogen atoms are evolved per chromium atom and with the foregoing conventional method, metal ion falls like a shower on the surface of the object to be plated, so the time between the reduction and the lattice assembly cannot be sufficiently long. In this reason, while the chromium layer to be deposited is grown as a crystal lattice with low atomic density, hydrogen is incorporated in the chromium layer. Consequently, the thicker the coating thickness is made, more likely cracking is to occur.
The present invention is, therefore, performed in consideration of the above described problems and provides a metal plating method for obtaining a metal plated film with good luster and excellent corrosion resistance and wear resistance.
According to one aspect of the present invention, a metal plating method can be provided for carrying out pulse plating by pulsed electrolysis by periodically applying electricity, wherein the above described pulsed electrolysis is carried out in condition that the pulse frequency and the current density are controlled so that the ratio of the quantity of deposited lattice per pulse to the height of the lattice is about 0.28 or lower, that the duty ratio of the above described pulse frequency is controlled to be 0.5 or lower, and that the duration of complete pause caused by distortion of pulse waveform is controlled to be one half or longer of the duration of current interruption.
xe2x80x9cthe ratio of the quantity of deposited lattice per pulse to the height of the latticexe2x80x9d is dimensionless number. Further, xe2x80x9cthe height of the latticexe2x80x9d indicates the height of lattice when the crystal face is oriented in the (111) face where the atomic density is highest.
xe2x80x9cthe ratio of the quantity of deposited lattice per pulse to the height of the latticexe2x80x9d is defined as xe2x80x9c(the quantity of deposited lattice per pulse)/the height of the latticexe2x80x9d and xe2x80x9cthe ratio of the quantity of deposited lattice per pulse to the height of the latticexe2x80x9d is sometimes only described to be the ratio of the quantity of deposited lattice.
The above described duty ratio means ti/(ti+t0) (refer to FIG. 2), wherein ti denotes a duration of pulsed current application and t0 denotes a duration of current interruption.
If the pulse waveform is an ideal one, the duration of current interruption t0 is equal to the time during which no current flows between the electrodes, however owing to the distortion of the waveform, the time during which current does not actually flows differs. Such non-application time during which current does not actually flows is called duration tk of complete pause as mentioned above.
According to the present invention, hydrogen is dispersed in the duration of current interruption t0 and suppressed from being incorporated in plated film, and the crystal face of a deposited metal can be controlled by controlling the reduced atom weight per pulse, thereby allowing for plating without cracking. The foregoing control of the reduced atom weight per pulse can be carried out by controlling the current density and the frequency.
According to the present invention, electroplating is carried out by pulsed electrolysis wherein hydrogen emitted from the cathode interface is dispersed far from the interface to lower the probability of absorbing hydrogen in crystal particles of chromium as well as to give arrangement of preferential orientation in high energy face, to prevent cracking, and to improve the wear resistance, ductility, and hardness of a plated film. In the case of chromium plating, which is of a body-centered cubic lattice, the crystal face is oriented in the (111) face where the atomic density is highest and the orientation ratio can be made to be 95% or higher by using the plating conditions as claimed in the present invention.
The numeral definitions of the present invention will be described below.
As described later, the relation between the quantity of deposited lattice per pulse and existence of cracks was studied using the pulse frequency as a parameter, and it was found that no cracking occurred (refer to table 1) in the condition that the ratio of the quantity of deposited lattice is 0.28 or lower (700 Hz or higher). Therefore, the ratio of the quantity of deposited lattice is 0.28 or lower (700 Hz or higher).
Meanwhile, the relation between the ratio of the quantity of deposited lattice and the pulse frequency is approximately same at current density used commonly for metal plating in a range of 10 to 1,200 A/dm2 if the current density is set up uniformly. When the ratio of the quantity of deposited lattice is 0.28, the frequency is 700 Hz and when the ratio of the quantity of deposited lattice is 0.22, the frequency is 900 Hz. Further, as described later, the surface roughness or the like of the plated film is stabilized and improved by controlling the frequency to be 900 Hz or higher, so it is preferable to control the ratio of the quantity of deposited lattice to 0.22 or lower.
If the duty ratio is controlled to be 0.5, cracking is stably suppressed, so that the duty ratio is set to be 0.5 or lower. The smaller the duty ratio is, the longer becomes the ratio of pause duration of the current application. Further, at the same current density, though the ratio of pause duration of the current application per pulse becomes shorter as the frequency increases, the quantity of electrolysis per pulse is lowered as well. In this case, there is no lower limit of the duty ratio. However, as the foregoing duty ratio is lowered, the pause duration of the current application becomes longer and that is effective for dispersion of emitted hydrogen and on the other hand, it takes a long plating time accordingly.
Also, the higher the current density is, the easier the pulse waveforms are distorted and the time (the duration of complete pause) during which practically no current flows becomes shorter than the pause duration (the duration of current interruption) of the current with ideal waveform. Taking that in consideration, the relation between the duration of current interruption and the duration of complete pause is studied and in the present invention, the duration of complete pause is regulated to at shortest one half of the duration of current interruption, because cracking occurred when the duration of complete pause is not more than one half of the duration of current interruption.
Next, the pulse frequency can be controlled to be about 900 Hz or higher.
According to the present invention, by controlling the pulse frequency to 900 Hz or higher, as noted above, the crystal particle diameter becomes small stably and the surface roughness is improved (refer to FIG. 5 and FIG. 7).
The method can be carried out by pulse plating while fluidizing a plating solution to be brought into contact with an object body to be plated at a flow velocity of about 0.04 (m/s) or higher.
By fluidizing the plating solution, dispersion of emitted hydrogen is promoted and hydrogen is more suppressed from being incorporated in the plated film.
The relation between the flow velocity of a plating solution and the crystal particle diameter of a plated film was studied and it was found that the crystal of the plated film is stably made fine and the orientation ratio of the foregoing high energy face is heightened by controlling the flow velocity to be 0.04 (m/s) or higher (refer to FIG. 3 and FIG. 4) and, therefore, the flow velocity is regulated to be 0.04 (m/s) or higher. Though the upper limit of the flow velocity is not specifically limited, it is preferable, in relation to the composition and the viscosity of the plating solution and the flow path of the plating solution in a plating bath, to keep the flow velocity sufficient to the extent within which no turbulent current such as swirling current is generated in the periphery of the object body in the fluidized plating solution.
The pulse frequency can be controlled to be about 900 Hz or higher and the following formula is satisfied when the ratio of the quantity of deposited lattice per pulse to the height of the lattice is Y and the pulse frequency is X (Hz).
xe2x80x83Yxe2x89xa6xe2x88x920.0932xc3x97ln(X)+0.8376
By specifying such a range, the corrosion resistance under high temperature is improved (refer to FIG. 15).